1. Field of the Invention
The present invention relates to a process and apparatus for producing an aqueous solution containing a mixture of chlorine dioxide and chlorine. In particular, the present invention relates to a process and apparatus for producing an aqueous solution containing chlorine dioxide and chlorine wherein the chlorine dioxide and chlorine are produced by the reaction of an alkali metal chlorate, an alkali metal chloride and a mineral acid wherein autodecomposition of chlorine dioxide is detected and controlled. Furthermore, the present invention relates to a process and apparatus for producing an aqueous solution containing chlorine dioxide and chlorine wherein the chlorine dioxide and chlorine are produced by the reaction of an alkali metal chlorate, an alkali metal chloride and a mineral acid mixture consisting of selected ratios of phosphoric acid to sulfuric acid in a specifically designed reaction zone so as to reduce or eliminate the likelihood of autodecomposition of chlorine dioxide.
2. Brief Description of the Prior Art
Aqueous solutions containing a mixture of chlorine dioxide and chlorine have found use in bleaching pulp, water treatment applications and other end uses. Furthermore, the chlorine dioxide and chlorine values in the aqueous solution may be separated and used individually for these and other applications.
One known method for making such chlorine dioxide and chlorine-containing solutions commercially has been to react in an aqueous medium an alkali metal chlorate, an alkali metal chloride and a mineral acid together to form simultaneously both desired chemicals. This known reaction is illustrated by the following reaction equation (A), wherein the preferred alkali metal chlorate has been sodium chlorate, the preferred alkali metal chloride has been sodium chloride and the preferred mineral acid has been sulfuric acid: EQU 2 NaClO.sub.3 +2 NaCl+2 H.sub.2 SO.sub.4 .fwdarw.2 ClO.sub.2 +Cl.sub.2 +2 Na.sub.2 SO.sub.4 +2 H.sub.2 O (A)
As can be seen, two moles of chlorine dioxide are produced for each mole of chlorine produced by this reaction. Sodium sulfate is also produced as a byproduct. In commercial processes, this sulfate byproduct has been either immediately separated from the ClO.sub.2 and Cl.sub.2 products or kept in the same aqueous reaction mixture for later end use processing.
This reaction has been taught in U.S. Pat. Nos. 2,863,722 (Rapson); 3,563,702 (Partridge et al.); 3,789,108 (Rapson); 3,816,077 (Fuller et al.); 4,414,193 (Fredette et al.); 4,534,952 (Rapson et al.); and in many other U.S. Patents.
There are many known variations of this reaction. For instance, HCl may be used instead of the alkali metal chloride For example, see U.S. Pat. Nos. 2,344,346 (Evans) and 3,933,987 (Schulz et al.). It is also known to employ methanol instead of the alkali metal choride For example, see U.S. Pat. No. 4,081,520 (Swindells et al.). It is also known to additionally use a bisulfate as a reactant. For example, see U.S. Pat. No. 3,733,395 (Fuller) It is also known to make SlO.sub.2 alone by reacting a chlorate herein by referencenin their entireties. with a mineral acid in the presence of a catalyst. For example, see U.S. Pat. No. 4,362,707 (Hardee et al.). All of the above-noted U.S. Patents are incorporated
While these above noted U.S. Patents generally teach that sulfuric acid is the preferred mineral acid reactant, some references also state that hydrochloric acid or phosphoric acid may instead be used. But, in fact, only U.S. Pat. Nos. 4,362,707; 4,381,290; 4,426,463; and 4,501,824 provide actual experimental data where H.sub.3 PO.sub.4 was used as the mineral acid instead of H.sub.2 SO.sub.4 in this type of reaction. See Example 8 of those four patents.
Moreover, some of these references make broad statements that mixtures of sulfuric acid and phosphoric acid may be employed as the mineral acid reactant for this particular reaction. For example, see column 5, lines 30-33; column 6, lines 69-71 and claim 1 of U.S. Pat. No. 3,563,702 (Partridge et al.); claims 1 and 5 of U.S. Pat. No. 3,816,077 (Fuller et al.); and column 3, lines 1-3 of U.S. Pat. No. 4,534,952 (Rapson et al.). But, in fact, no teaching has been found where a mixture of sulfuric acid and phosphoric acid was ever actually used for this type of reaction.
One problem reported with the reaction of Equation (A), above, and similar reactions, is chlorine dioxide "puffing" or spontaneous autodecomposition in the reaction vessel. While the reason why this unwanted decomposition occurs is not exactly known, it is believed to be tied to the reaction temperature and presence of impurities or formed byproducts in the reaction mixture. This "puffing" is undesired because it causes a decrease in the yield of the ClO.sub.2 product and, if great enough, explosions which may damage equipment or shut down processes or both. Also, it has been found that once this ClO.sub.2 "puffing" begins, it may continue to proceed unless the reaction conditions in the reaction vessel are changed.
The prior art apparatus for carrying out the reaction of equation (A), above, or similar reactions generally comprised large slender cylindrical columns (e.g. several feet high with much narrower diameters). The undesired ClO.sub.2 puffing or autodecomposition was generally detected by temperature monitors located in the reaction vessel and the puffing was generally corrected by passing a gas purge (e.g. air or an inert gas) through the reaction vessel. Such detection means and correction means are not totally suitable for relatively small generation vessels where the system must react immediately to the occurrence of a puffing event or otherwise risk an explosion within the generation vessel or complete loss of ClO.sub.2 generation or both.
Accordingly, there is a need in the art for a process and an apparatus for producing an aqueous mixture of ClO.sub.2 and Cl.sub.2 by reacting an alkali metal chlorate, alkali metal chloride and a mineral acid together where there is both a reduced likelihood of a chlorine dioxide "puffing" event and, if such an undesirable event should occur, a method for quickly detecting and automatically correcting such undesirable events. The present invention does present a solution to that need.